Electrical stimulation of skeletal muscle can be used to assist individuals paralyzed due to central nervous system dysfunction in performing functional movements. Muscle fatigue, however, is a major limitation in the practical use of this approach. Previous work has shown that patterns of activation using variable-frequency trains (VFTs) that exploit the catchlike property of skeletal muscle produce greater forces than traditionally used constant-frequency trains (CFTs), particularly in fatigued muscle. The proposed studies will extend these previous findings and will provide information that is needed to identify stimulation patterns that maximize force and minimize fatigue under a variety of physiological conditions. The overall goal of this work is to identify stimulation patterns that maximize performance during repetitive, electrically elicited, contractions in skeletal muscle. The specific aims of the project are 1) To identify the combination of stimulation trains that enable human skeletal muscles to produce the greatest number of repetitive, isometric contractions that reach a targeted force level, 2) To identify the combination of stimulation trains that enable human skeletal muscles to produce the greatest number of repetitive, non-isometric contractions that achieve a targeted level of excursion, 3) To extend our understanding of variable frequency train stimulation to include its effects on the forces and fatigue produced from muscles of subjects with spinal cord injuries, and 4) To continue the development and testing of mathematical models that predict the force output and fatigue produced in response to a wide range of stimulation frequencies and patterns. These models will be used to facilitate the identification of the optimal pattern of stimulation for each physiological condition and task that will be studied. The identification of stimulation patterns that maximize muscle performance will allow clinicians to select more physiologically advantageous activation patterns to use during functional electrical stimulation.